R 查找具有相同值的所有行的总和。

我有一个数据框，看起来像这样-

>df
words num 
his    92   
his    91   
there  91   
you    90   
who    90   
come   89   
you    70   

现在，我想聚合常用词，并对它们的

num

列求和。所以我的最终输出会是-

words   num
his     183   
there   91   
you     160   
who     90   
come    89   

---

我在R怎么做？我知道这件事很简单，但我无法理解

转换为

数据。表

：

setDT(df)
df[, sum(num), by=words]
   words  V1
1:   his 183
2: there  91
3:   you 160
4:   who  90
5:  come  89